Globally, the single largest source of CO2 emissions is power generation. In the U.S., electric power accounts for 38% of CO2 emissions, followed by transportation at 33%. It is estimated that about 20% of the U.S. electric power generation is for air conditioning.
The primary use of air conditioning is cooling. Air conditioning drives the peak electrical load during warm months.
Every electric grid is supplied by three types of power plants for meeting the changing power demand during the day. These power plants (or modes of operation) are referred to as base load, intermediate load, and peak load. Base load is the electricity that is continuously supplied to the grid—whether it is used or not. When most residences and commercial buildings have their lights off, people are in bed, TVs and computers are in “off” or “hibernate” mode, base load electricity generators run in the background at a constant generating capacity. Power plants that supply base load electricity to the grid are typically among the most efficient due to their steady mode of operation. These include hydroelectric and nuclear power plants.
Around 6 a.m., as people wake up and work restarts in offices, stores, and factories, the intermediate load generators start supplying power to the grid in response to the daytime demand. Intermediate load power plants include mainly fossil fuel fired boilers and turbines. However, these also include renewable power sources such as wind and solar that, due to their intermittent nature, cannot be relied upon for base load.
Later in a typical warm day, as temperatures rise and air conditioning units need to stay on to keep work and home environments at a comfortable temperature range, additional generators need to come online to meet this added demand. In the U.S., the peak load generators are mainly natural gas fired. Many other countries rely on diesel generators to meet peak electricity demand. Peak demand in summer time is in late morning and the afternoon. The winter peak is generally a lower load value and is reached later in the day (mainly for evening space heating, primetime TV viewing, running laundry and dishwashers, etc.). Base, intermittent, and peak operations, make up about 50-60%, 20-30%, and 20-30% of total daily electrical generation capacity, respectively. Base and intermediate power are also referred to as “off-peak” electricity.
Based on the preceding overview, it is clear that the effects of CO2 emissions from combustion of fossil fuels can be mitigated by reducing the peak cooling demand for air conditioners during warm days. Phase Change Material (PCM) having a solid-liquid phase transition (i.e., melting and freezing) temperature of 21-26° C. (70-79° F.) and relatively high latent heats of phase transition (>60 J/g), have been proposed as thermal storage devices that can effectively shift the peak load energy requirements to base load power generating periods when capacity typically exceeds demand.
For example, as described in the prior art, PCM can be incorporated into wall boards and ceiling tiles of a room. As the daily temperature rises, the PCM starts to melt. The temperature of the PCM remains essentially unchanged while it is melting, maintaining the indoor temperature in the comfort range of 21-26° C. without need for peak load air conditioning. Once all the PCM has melted, the room temperature rises, the air conditioner starts, and the PCM starts to refreeze using off-peak (i.e., base load) electricity.
One can look at the shifting of electrical demand from peak to base load in terms of storing heat or cold. The PCM latent heat is its effective thermal energy storage capacity. When a PCM, which is liquid at outside daytime temperature, is frozen at nighttime using off-peak air conditioning, it becomes a cold storage device. This cold is recovered the next day (as the PCM undergoes phase change) to help keep the inside temperature at the comfortable range without use of peak load air conditioning. Here, the PCM acts as a natural thermostat, much like ice cubes maintain the temperature of a beverage constant at around 0° C. while melting therein.
The Coefficient of Performance (COP) for an air conditioning unit is given by Equation 1, where Tamb and Told are the ambient and cooled media temperatures, respectively.COP=Tcold/(Tamb−Tcold)  (1)As observed from Equation 1, the lower the ambient temperature, the higher the COP and hence, the more efficient the air conditioner's operation. Operating at higher efficiency is another advantage of shifting the cooling load from hot days to cooler nights.
In winter time, the same transition stores solar energy during the day for recovery on cold nights. The same PCM, now in solid phase at outside nighttime temperature, acts as a heat storage device to store solar energy and reduce fuel consumption after sundown. Presence of PCM in the room and its surroundings also helps to dampen the oscillating temperature control profile associated with most thermostats.
To summarize, PCM systems can store heat (e.g., solar energy), store cold (e.g., off-peak air conditioning), and control temperature (by undergoing phase transition within the desired temperature range). As such, their innovative and cost-efficient deployment in buildings and other dynamic thermal systems can decrease CO2 emissions associated with heating and air conditioning, improve thermal efficiency of buildings, and reduce size/cost of air conditioning units.
Refrigerants, typically fluorinated hydrocarbons, are potent greenhouse gases themselves. Use of smaller air-conditioning units translates to lower amounts of refrigerant emissions.
PCM compositions include any component having the desired phase transition temperature and a relatively high corresponding latent heat value. Both inorganic and organic compositions are cited in prior art, such as salt hydrates and paraffins. Paraffins are considered particularly suitable because of their low toxicity, material compatibility, thermo-oxidative stability, and water repellent properties. One disadvantage of paraffins is their flammability. Even though the flash points of most PCM-range paraffins are well above 100° C., they can fuel a fire.
Many PCM applications require the material to retain its shape whether the phase changing component is in solid or liquid phase. The prior art describes microencapsulation and macroencapsulation methods to produce such “form stable” or “shape stable” PCMs. Microencapsulation is the encapsulation of solid or liquid particles of 1 micron to 1 mm diameter within a solid shell. Although physical processes like spray drying have been proposed, one method for production of shell-core PCM microcapsules is a batch chemical process known as coacervation.
Coacervation is the formation of two liquid phases—typically a polymer and another organic liquid. This was the basis for the classic method developed by National Cash Register (NCR) for carbonless copy paper as well as many other applications, including controlled release agricultural and pharmaceutical products. In the first step of microencapsulation by coacervation, a gelatin is dissolved in water to form a “sol,” which is then heated to solubilize the gelatin. Then the PCM paraffin is dispersed in the aqueous gelatin solution at a temperature range of 40-60° C. At this temperature range, the solution of the shell material is liquid. The suspended paraffin droplet size is controlled by agitation/stirring rate and/or use of surfactants. The pH is then adjusted (typically to 4.0-4.5 range) to cause the gelatin to deposit around the core paraffin material. At this point, the system is cooled to around room temperature, for the hardening of the shell. This can be done by adding a cross-linkable water soluble polymer such as urea-formaldehyde. The pH is then raised to 9.0-11.0 range by addition of a sodium hydroxide solution. The PCM microcapsule slurry is then cooled down further to the 5-10° C. range and maintained at that temperature for 2-4 hours. The microcapsules are then filtered or centrifuged and dried to yield a PCM product with a powder-like appearance.
The small batch operation, provisions for handling the toxic polymers/initiators, as well as water treatment costs, make PCMs produced by this process expensive. These microcapsule PCMs command a high price and target low volume specialty product markets. Furthermore, the small size of the microcapsules prevents them from being used as PCM devices by themselves and are therefore incorporated into other articles, adding another step in the manufacturing process.
Building products that act as “carriers” for the PCM microcapsules are typically wallboards (e.g., gypsum boards) and ceiling tiles. However, since the PCM microcapsules are trapped within the wallboard, heat transfer is conduction/radiation limited and not driven by the more efficient natural or forced convection mechanisms (e.g., turbulent flow of air around the PCM particle). This limits the overall heat transfer coefficient between the PCM and the air within the room, and since there is only about 6-8 hours of off-peak air conditioning available for removing all the heat stored during the day, the effectiveness of the PCM system is similarly limited.
Macroencapsulation involves filling plastic containers resembling small yogurt or Jell-O® cups with the PCM. These macroencapsulated PCM devices can be fabricated in automated production lines making final products resembling sealed dimpled sheets or large bubble wraps. Macroencapsulation is commonly used for inorganic PCMs. However, several technical issues have limited their use for organic PCMs. One issue is solubility (and hence incompatibility) of most common macroencapsulation material, like low density polyethylene (LDPE) in paraffins. Use of metal containers has also been proposed for macroencapsulated PCMs. However, these have the disadvantage of being too heavy and costly for most PCM applications. Another disadvantage of macroencapsulation is that during the freezing of the paraffin melt, the wall cools before the bulk of the paraffin. As a result, a wax film can form on the inside walls of these macroencapsulation storage units that reduces the rate of heat transfer from the molten paraffin mass within. This can lead to significantly reduced thermal storage performance.
The prior art also describes PCM composites that may be converted into pellets. For example, some polymer composites include a cross-linked high-density polyethylene powder (HDPE) and a PCM paraffin. These patents teach the use of cross-linked HDPE and ethylene-vinyl acetate (EVA) copolymers to provide form-stable “non-exuding” PCM composites. Exuding or “oozing” of paraffin from conventional HDPE is a problem. However, cross-linked HDPE is costly, while EVA reduces the composite's melt viscosity, making conventional pelletizing difficult and limiting the amount of PCM paraffin that can be incorporated. Finally, such methods do not address the issue of organic PCM flammability.
There is thus a need for products, methods, and apparatuses for effective utilization of PCM in applications that can reduce peak cooling load. Furthermore, these need to be of simple design, low cost, and suitable for wide deployment such that peak load electricity demand and corresponding CO2 emissions are significantly reduced. It is to such products, methods and apparatuses that the present disclosure is directed.